EXISTSCount how many of the given keys exist.
| 構文 | EXISTS key [key ...] |
| グループ | generic |
| 引数の数 | -2 |
| 導入バージョン | 1.0.0 |
| フラグ | readonly |
| 計算量 | O(N) keys, one O(1) probe each |
| Redis 互換性 | differs: the multi-key form is gathered per shard, so the count is not a point-in-time snapshot across shards |